1. Field of the Invention
The present invention relates to a Doherty amplifier; and; more particularly, to a Doherty amplifier capable of maintaining linearity and high efficiency with reducing the size of a circuit and simplifying the structure thereof.
2. Background of the Related Art
Generally, a modulation signal having a high peak to average power ratio (PAPR) is used for transmitting a great deal of information, and this causes a power amplifier to considerably consume the current and reduces efficiency thereof remarkably. A Doherty method is used as one of methods to solve such problems, and it is widely applied to a power amplifier for a base station, a repeater, a portable terminal or the like by improving the efficiency in a wide output power range.
Such a Doherty amplifier is composed of an output stage unlike that of a conventional amplifier, and FIG. 1 is a diagram showing the exemplary general output stage of a conventional Doherty amplifier. Referring to the FIG. 1, the Doherty amplifier has a main amplifying unit (or carrier amplifier), an auxiliary amplifying unit (or peaking amplifier) for optimal output capability and an individual matching network unit to each of the amplifiers, wherein the main amplifying unit and the auxiliary amplifying unit are connected through a microstrip transmission line with a length of λ/4 (hereinafter, referring to as ‘λ/4 line’). At this time, an offset line may be constructed by being installed between the two amplifiers and the λ/4 line for the correct operation of the Doherty amplifier.
The λ/4 line modulates the load impedance of the main amplifying unit according to an input drive level, and the offset line plays the roles of allowing the main amplifying unit to modulate the load impedance exactly in low input drive level and the auxiliary amplifying unit to prevent power leakage. In the result, the improved efficiency and linearity are obtained in comparison with a general balanced amplifier.
Meanwhile, when the Doherty amplifier is applied to a base station, the stringent linearity of a base station is satisfied by using feedforward or digital predistortion.